1. Field of the Invention
The invention relates in general to a method of optical proximity correction (OPC). More particularly, to a method of forming an assist feature, a serif, or a hammerhead on an original pattern, or locally reducing or enlarging the original pattern to reduce optical proximity effect (OPE), so as to optimize the fidelity of the original pattern.
2. Description of the Related Art
In the very advanced fabrication technology of integrated circuits, to reduce the dimensions of devices and to increase the integration are a leading trend and topic for further development. The photolithography and etching step is one of the most crucial factors to determine the reliability and performance of devices and the integrated circuit.
Commensurate with the size reduction, various process limitations have made the photolithography more difficult. The increased integration causes the devices shrinkage and a reduced distance between devices. A deviation is thus easily caused while transferring a pattern from a photo-mask to a layer on a wafer. For example, after being transferred from a photo-mask to a photo-resist layer on a wafer, a right angle corner of the pattern on the photo-mask is often rounded, the line ends would become short, or the line widths increase/decrease, etc. Such effects of deviation are referred to as the optical proximity effect (OPE).
In the integrated circuit of a substantially low integration, these deviations would not greatly affect the performance and operation of devices. However, in the integrated circuit with a high integration, this effect may cause a serious problem. For example, when the line width expands unexpectedly, one metal line may overlap on the other metal line to cause a short circuit, so as to cause a device failure. In other words, the enhancement of operation speed corresponding to the shrinkage of devices is gradually limited due to lack of fidelity for transferred pattern during photolithography processes. An example of optical proximity effect is shown as FIG. 1, in which each corner of a rectangle is rounded, while the linewidth is expanded.
The causes of the optical proximity effect include optical factors (interference between light beams transmitted through adjacent patterns), resist processes (baking temperatures/times, development time, etc), the reflection of light from and irregularity of the substrate. The mainstream of preventing pattern transfer degradation due to the OPE is to add corrections allowing for degradation to a mask in advance. This process is referred to as optical proximity correction (OPC).
Different approaches have been made for the optical proximity correction, for example, U.S. Pat. Nos. 5,732,233 and 5,879,844 to Grazal et al. and Yamamoto et al., respectively, disclose methods of optical proximity correction. In their disclosures, an assist feature is added onto a main pattern, or the linewidth of the main pattern is increased/decreased to increase the contrast for exposure or to enlarge the process window.
Grazal et al. performs an exposure on a test pattern first. The transferred test pattern is then compared to a main pattern to obtain a deviation between these two patterns to determine whether an optical proximity correction is to be performed. In addition, the correction degree can be determined according to the level of deviation.
Referring to FIG. 2, Yamamoto uses a wafer having a conductive layer (oblique lines) on a diffusion layer (dot area) as an example. In this figure, the parallel edges (the bold lines) of the overlapping part of these two layers are the part to be corrected. In addition, while a distance between two neighboring edge lines are narrower than a threshold value, or if a ratio of the length of an edge line to the distance is less than another threshold value, these edges are to be corrected.
By the above conventional method, an empirical result is required for determining whether an optical proximity correction is to be performed on a pattern. Or alternatively, a relationship between two adjacent layers formed on a wafer has to be obtained. Therefore, it causes a great trouble in data processing.
Accordingly, the invention provides a method of optical proximity correction. Using the critical dimension (CD) of an original main pattern as a determinant of whether an optical proximity correction is performed on the original main pattern, neither an empirical result nor a relationship between two adjacent layers is required. Therefore, the data handling is simplified while fidelity of a transferred pattern is retained.
In the invention, an original pattern to be transferred is provided. The critical dimension of the original pattern is obtained. When the critical dimension is less than or equal to 2.5 times of a wavelength of an exposure light source, serifs are added on each corner of the original pattern, or a hammerhead is added onto each trunk of the pattern. If the critical dimension is less than or equal to the wavelength, an assist feature is added into the original pattern. The original pattern is thus corrected for forwarding an exposure step of a photolithography process. In addition, by correcting the original in a sequence of adding the serifs and hammerhead in front of adding the assist feature, an overlap in the corrected pattern or an overly reduced distance is avoided.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.